1. Field of the Invention
The invention relates to a method of operating a furnace, in particular a method for bringing a plurality of steel slabs to rolling temperature in a furnace with controllable energy supply. The method is particularly applicable in hot-strip mills.
2. Description of the Prior Art
Furnaces, continuous reheating furnaces or walking beam furnaces used in hot-strip mills usually have three heating zones, namely a charging zone, a central zone and an end zone. Each of these zones has a controllable energy supply. A typical furnace can have a furnace charge of thirty-eight steel slabs of about one metre width, of which at any time fourteen are in the charging zone, ten in the central zone and the rest in the end zone. Every three to five minutes a new steel slab is fed into the charging zone, and a steel slab which is at rolling temperature leaves the end zone, all according to the known "first in, first out" principle.
A steel slab is at rolling temperature when it has passed through a curve or pattern of temperatures, from its initial temperature on being charged into the furnace, in such a way that the steel slab is well heated through and the outermost layers of the steel slab are not over-heated. That is to say, the core of the steel slab must have reached a desired temperature which in principle is the same temperature as the outermost layers of the steel slab. However, on account of the heat transfer needed from the outside of the steel slab towards the core, it is permissible and necessary to have a variation in the temperature level of these outermost layers. Too low a temperature at the upper side of the steel slab creates undesirable curling up phenomena of the steel slab as a result of cooling. Even so the temperature on the outside of the steel slab must remain within tight limits in order, among other reasons, to hinder oxidation on the surface of the steel slab.
One conventional method of controlling a furnace consists of specifying a desired temperature level of the gas in the furnace in each zone, the levels being related to the desired curve of temperatures for each slab. The energy supply into each zone is dependent on the temperature level in each zone at any time. See U.S. Pat. No. 4,501,522 for a particular description of a method of this general kind.
This known method is satisfactory if there is stable or steady state operation of the hot-strip mill and of the furnace. Disturbances in the so-called "pulling speed" that is to say the frequency at which a steel slab is taken out of the end zone of the furnace in order to be rolled out, may still be accommodated in the known method, when those disturbances are not too massive. However, it is different where these disturbances becomes greater, for example as a result of interruptions in rolling out, leading to the furnace being run at a reduced level for a longer period.
Even more significant are disturbances resulting from varying starting conditions of the steel slabs. The existing method is unable to deal with these satisfactorily. In particular, a problem arises when the starting temperature of subsequent steel slabs differ from one another.
This problem has gained particular urgency because of the rise in production of continuously cast slab material. From the point of view of operating economy and for energy reasons, there are advantages in further processing such steel slabs in the hot-strip mill as quickly as possible after the continuous casting. It is advantageous for operating economy because in this way holding of interim stocks between the continous casting plant and the hot-strip mill is avoided, and the throughput time from the start of the continuous casting and the end of rolling is reduced. It is advantageous for energy reasons because less energy is needed to bring hot steel slabs charged directly into the furnace of the hot-strip mill up to the desired rolling temperature.
In practice, however, the entire furnace charge may not consist of directly charged continuously cast material. In practice the hot-strip mill furnace is charged both from a store holding a stock of steel slabs cooled to ambient temperature and with steel slabs still hot from the continuous casting process. These hot steel slabs having a temperature of 400.degree.-600.degree. C. These steel slabs and the steel slabs with a temperature of about 20.degree. C. must all be heated up to about 1200.degree.-1260.degree. C.
U.S. Pat. No. 4,338,077 describes one method of attempting to deal with this problem. The temperature patterns are controlled in dependence on the position of a boundary material, which is the first material of a group of hot or cold slabs. The present invention is based on a different concept.